The invention relates to an electrical interface circuit for isolating a plurality of monitored voltage inputs from an indicating circuit and, more particularly, relates to an interface circuit for use between a brush wear indicating circuit and a plurality of brush wear sensors.
The use of various sensor devices and indicating circuits to monitor and signal the occurrence of given degrees of brush wear, as such wear occurs during operation of a dynamoelectric machine containing such brushes was generally well-known before the present invention. A number of different types of such brush wear indicator devices are disclosed, for example, in my co-pending U.S. Patent Application Ser. No. 06/183,920, which was filed on Sept. 4, 1980 and is assigned to the assignee of the present invention. Moreover, a number of earlier brush wear indicating devices were described in several of the patents referred to in that co-pending application. Further specific examples of known prior art brush wear indicating sensor devices will be given below in conjunction with the description of the present invention in order to fully explain the best mode of its operation now known to the inventor.
In addition to the large number of different types of brush wear indicating devices known in the prior art, there presently exists a considerable number of different forms of indicating or alarm circuits which have been found to be particularly suitable for use with brush wear indicator devices. Several such circuits are shown, for example, in U.S. Pat. No. 3,523,288, which issued on Aug. 4, 1970. As is explained in the disclosure of that patent, it is generally well-known to provide brush wear indicating circuits that provide either normally-on, or normally-off indicating alarm means that are switched from one state to another responsive to the occurrence of a predetermined degree of brush wear in a monitored brush.
Another U.S. Pat. No. 4,024,525, which issued May 17, 1977 discloses a brush wear indicator circuit that is operable to simultaneously monitor wear occurring in a plurality of brushes, and to indicate the occurrence of a predetermined degree of such wear on one or more of the brushes by actuating a single indicator light or alarm signal. Selective actuation of the indicator light is controlled by a control circuit connected between brush wear sensor probes and the indicator light.
A common problem encountered in the application of presently known brush wear indicator devices and associated indicating alarms or signal circuits is that the input signals transmitted to the indicating circuits are often unreliable or include undesirably wide swings of input voltage. In addition, many prior art brush wear indicator devices are limited by their nature to utilization of current conducted through a monitored brush. Such brush currents often become very erratic when the brush wears sufficiently to cause it to have uneven contact with a commutator on which the brush is riding. Moreover, in brush wear indicator circuits that rely on a monitored brush voltage to actuate an indicating signal, either voltage-sensitive components in the coupling circuit between the brush wear sensor and the indicating circuit, or the indicator signal means itself may be damaged due to peaks of high voltage occurring on the monitored brushes. In view of these common problems with some of the prior art brush wear indicator devices and circuits, it is desirable to provide an interface circuit for use between brush wear sensor devices and brush wear indicating circuits to provide an efficient and reliable means for both manufacturing new brush wear indicator systems and for retrofitting existing brush wear indicator systems to avoid some of those problems.